Alumina ceramic used in technical processes common contains more than approximately 92 wt. % alumina and less than approximately 8 wt. % of an auxiliary sintering agent, such as silicon dioxide, titanium dioxide, magnesium oxide, or calcium oxide. In most cases a 96 wt. % alumina ceramic is used, which thus contains 4 wt. % auxiliary sintering agent and whose sintering temperature is approximately 1,500.degree. C.
In special cases, the alumina ceramic may also be particularly pure (99.5 wt. % alumina), i.e., the ceramic contains only an extremely small percentage of an auxiliary sintering agent; the sintering temperature is then approximately 1,600.degree. C.
A special application of such alumina ceramic are ceramic pressure sensors. Such sensors, as is well known, comprise a ceramic substrate and a ceramic diaphragm which is jointed to the substrate near its edge to form an internal chamber.
As a material for joining the sintered alumina ceramic bodies, U.S. Pat. No. 4,177,496 describes a glass frit, and U.S. Pat. Nos. 5,005,421 or 5,334,344 an active brazing solder.
Because of the more or less differing thermal expansion coefficients of glass frit or active brazing solder and alumina ceramic, both materials pose problems, particularly if an optimum match is required over a wide temperature range. Although active brazing alloys can be found where this match appears satisfactory, cf. the U.S. Provisional Application Ser. No. 60/023,079 filed Aug. 02, 1996 and the copending U.S. Non-Provisional Application Ser. No. 08/896,481 filed on Jul. 18, 1997, the need for optimally matched thermal expansion coefficients of alumina ceramic and joining material still exists.